The use of an exhaust gas recirculation (EGR) system to recirculate internal combustion engine exhaust gas to an inlet air path of an engine is well known. EGR can lower the level of certain undesirable engine emission components such as nitrogen oxide (NOx) and can improve fuel economy. Up to a limit, NOx emissions decrease with increasing EGR levels. Beyond the limit, EGR can increase formation of other undesirable engine emission components and can reduce vehicle drivability.
EGR typically involves recirculation of exhaust gas through an EGR passage between an engine exhaust conduit and an engine fresh air intake passage. A valve within the EGR passage (the EGR valve) is controlled to vary a restriction within the EGR passage to regulate the flow of exhaust gas therethrough. When EGR is not required, the EGR valve is driven to a full restriction (closed) position, typically through a spring preload. The spring preload is commonly required to be substantial to ensure rapid closing of the EGR valve when necessary, and to ensure proper sealing of a closed EGR valve. When EGR is required, the EGR valve is driven to an open position through application of a position control signal to an actuator mechanically linked to the EGR valve. The degree of opening of the EGR valve varies with the magnitude of the position control signal.
When the EGR valve is open, recirculated exhaust gas enters the fresh air intake passage and flows to the engine cylinders. For optimum performance, the exhaust gas should mix thoroughly with the fresh air so that each cylinder receives substantially identical proportions of fresh air and exhaust gas. Typically, the exhaust gas is added to the fresh air immediately prior to entering an intake manifold to minimize the response time between a signal commanding the EGR valve to open and exhaust gas reaching the engine cylinders.